This application claims priority to an application entitled xe2x80x9cApparatus and Method for Reducing Nonlinear Distortion in an Automatic Gain Control Systemxe2x80x9d filed in the Korean Industrial Property Office on Oct. 27, 1999 and assigned Serial No. 99-46847, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for reducing nonlinear distortion in a system having an automatic gain control function.
2. Description of the Related Art
In general, a mobile communication system has an automatic gain control (AGC) function to automatically control a gain of a transceiver depending on a power level of a received signal. During transmission, the mobile communication system with the AGC function (hereinafter, referred to as an AGC system) properly controls a gain of a transmission power amplifier to transmit signals at a proper transmission power level in response to a transmission power control command uniquely transmitted to each mobile station from a base station in order to prevent performance degradation of the system due to interference with other mobile station users. During reception, the AGC system properly controls a gain of a receiving amplifier to compensate for a fading phenomenon which is a time-dependent variation of reception power due to a variation in the distance from the base station and the movement of vehicles or pedestrians.
FIG. 1 illustrates a structure of an AGC system according to the prior art, which is disclosed in U.S. Pat. No. 5,257,283, entitled xe2x80x9cSpread Spectrum Transmitter Power Control Method and Systemxe2x80x9d.
Referring to FIG. 1, a signal received from an antenna is applied to an intermediate frequency amplifier (IF AMP) 94 through a down-converter 90 and a bandpass filter (BPF) 92. The output of the IF amplifier 94 is applied to a baseband processor 108 through an analog-to-digital (A/D) converter 106 and is also applied to an AGC detector 96. The AGC detector 96 has two different uses, as follows:
(1) During reception, the AGC detector 96 generates a control voltage for controlling a gain of the IF amplifier 94 to maintain a level of the received signal.
(2) During transmission, the output of the AGC detector 96 is compared with a reference power level value generated by a control processor 110 in order to adjust transmission power to a value set by a mobile station power control command transmitted from the base station. The reference power level value is applied to a comparator 98 through a digital-to-analog (D/A) converter 112. The output of the comparator 98 is applied to a nonlinear filter 100, the output of which is nonlinearly varied according to a variation rate in the output of the comparator 98. The nonlinear filter 100 has the function of setting an increasing rate of a transmission power control voltage to be less than a decreasing rate thereof in order to prevent performance degradation of the system due to an abrupt increase in the transmission power. In addition, during a transmission mode of the mobile station, an IF amplifier 104 controls a signal received from a transmission modulator (not shown) depending on a control signal generated from the control processor 110. Here, a D/A converter 114 is required to generate a control voltage for controlling a gain of the IF amplifier 104. An output signal of the IF amplifier 104 is applied to an IF amplifier 102 which also receives the output of the nonlinear filter 100. In this manner, the AGC system enables automatic gain control during transmission.
In the AGC system shown in FIG. 1, when a high-power input signal is applied to a first stage in a receiving stage, several elements located in an RF (Radio Frequency) stage are saturated causing nonlinear distortion of the received signal. Finally, this distorted signal is applied to an AGC loop, and when the baseband signal is demodulated in a modem, clipping distortion is further added to the distorted signal by the AGC loop, causing degradation of demodulation performance. The same problems occur even during a transmission mode. That is, if transmission power is controlled using only the transmission AGC amplifier when increasing the transmission power, the RF elements in the next stage can be easily saturated due to the high power of the AGC amplifier in the transmission stage. Therefore, for the high-power reception and transmission signals, it is necessary to reduce distortion of the signals input to and output from the AGC loop; otherwise the nonlinearly-distorted signals input to and output from the AGC loop are transmitted to the next stage, causing degradation of the overall system performance.
Of course, when it is necessary to transmit a high-power signal by providing the nonlinear filter 100 in the transmission stage as shown in FIG. 1, it is possible to prevent the AGC amplifier in the transmission stage from being saturated. However, it is not so easy to realize such a nonlinear filter 100 having the proper features. Moreover, in order to control a gain of the IF amplifier 102, an additional circuit is required to convert the output level of the nonlinear filter 100 to a gain varying control voltage for the IF amplifier 102. Also, it is not easy to construct such a level conversion circuit because of the nonlinear characteristics of the nonlinear filter 100.
It is, therefore, an object of the present invention to provide an apparatus and method for reducing nonlinear distortion of transmission/reception signals in an AGC system.
It is another object of the present invention to provide an apparatus and method for preventing a signal input to an AGC loop in a receiving stage from being distorted by an RF stage when a high-power input signal is applied to a first stage of the receiving stage in an AGC system.
It is yet another object of the present invention to provide an apparatus and method for preventing a signal output from an AGC loop in an IF stage from being distorted, when an output signal of a transmission stage has a high power level in an AGC system.
It is still another object of the present invention to provide an apparatus and method for preventing RF elements from being saturated for high-power transmission/reception signals in an AGC system.
It is still another object of the present invention to provide an apparatus and method for preventing degradation of system performance for the high-power transmission/reception signals in an AGC system.
To achieve the above objects, an apparatus for adaptively controlling an output level of a local oscillator is provided to reduce nonlinear distortion which may occur when receiving a high-power RF signal or when it is necessary to transmit a high-power transmission signal. When a high-power RF signal is applied to a first stage of the receiving stage, the apparatus prevents the signal input to a reception AGC loop from being distorted by decreasing an output level of the local oscillator being provided to a down-converter. Further, when the output signal of a transmission stage has a high power level, the apparatus decreases the output level of a transmission AGC amplifier to some extent and instead, increases the output level of the local oscillator being input to an up-converter located in an IF stage so that it is possible to obtain a nonlinear distortion-reduced signal while obtaining the same output level. As a result, the invention expands the linear operation region of the transmission/reception AGC loop, so that it is possible to prevent degradation of the system performance resulting from the limitation on the linear operation region of the AGC loop due to nonlinear distortion.
According to a first aspect of the present invention, an apparatus is provided for reducing nonlinear distortion occurring in a down-converter according to a level of a received RF signal in a receiving stage of a mobile communication system including a low-noise amplifier for low-noise amplifying an RF input signal received through an antenna and outputting the received RF signal, said down-converter for down-converting the received RF signal to an IF signal by mixing the received RF signal with a reference frequency signal, and an AGC amplifier for amplifying the IF signal according to an AGC voltage to maintain power of the IF signal and outputting the amplified signal for demodulation. The apparatus includes a power detector for detecting power of the received RF signal; a comparator for comparing the detected received RF signal with a predetermined reception power threshold and generating a comparison control voltage being inversely proportional to a difference therebetween; and a voltage controlled amplifier for amplifying the reference frequency signal according to the generated comparison control voltage and outputting the amplified reference frequency signal to the down-converter.
According to a second aspect of the present invention, an apparatus is provided for reducing nonlinear distortion occurring in an AGC amplifier due to an increase in gain thereof when a power level of a transmission signal is high in a transmission stage of a mobile communication system including the AGC amplifier for amplifying an IF transmission signal according to a transmission signal power level control voltage to maintain power of modulated transmission signal, an up-converter for mixing an output signal of the AGC amplifier with a reference frequency signal to up-convert the IF signal to an RF signal, and a power amplifier for power amplifying the RF signal and transmitting the amplified RF signal. The apparatus includes a comparator for comparing the transmission signal power level control voltage with a predetermined transmission power threshold and generating a comparison control voltage being inversely proportional to a difference therebetween; and a voltage controlled amplifier for amplifying the reference frequency signal according to the generated comparison control voltage and outputting the amplified reference frequency signal to the up-converter.
According to a third aspect of the present invention, there is provided an apparatus for reducing nonlinear distortion of transmission/reception signals in a mobile communication system having an AGC function. The apparatus includes an oscillator for oscillating a reference frequency signal; a low-noise amplifier for low-noise amplifying an RF input signal received through an antenna and outputting a received RF signal; a power detector for detecting power of the received RF signal; a first comparator for comparing the detected power of the received RF signal with a predetermined reception power threshold and generating a reception comparison control voltage being inversely proportional to a difference there between; a first voltage controlled amplifier for amplifying the reference frequency signal according to the reception comparison control voltage; a down-converter for mixing the received RF signal with an output of the first voltage controlled amplifier to convert the received RF signal to an IF signal; a reception AGC amplifier for amplifying the IF signal according to an AGC voltage to maintain power of the IF signal and outputting the amplified signal for demodulation; a controller for determining the reception power threshold, a transmission power threshold and a transmission signal power level control voltage; a transmission AGC amplifier for amplifying the demodulated transmission signal according to the transmission signal power level control voltage to maintain power of the demodulated transmission signal; a second comparator for comparing the transmission signal power level control voltage with the transmission power threshold and generating a transmission comparison control voltage being inversely proportional to a difference there between; a second voltage controlled amplifier for amplifying the reference frequency signal according to the transmission comparison control voltage; an up-converter for mixing an output of the transmission AGC amplifier with an output of the second voltage controlled amplifier to convert the output signal of the transmission AGC amplifier to an RF transmission signal; and a power amplifier for power amplifying the RF transmission signal and transmitting the amplified transmission signal through the antenna.